In motor vehicle cooling systems, viscosity couplings, that is to say liquid friction couplings, are employed, to couple driven parts of the cooling system, in particular a fan, to the vehicle engine and drive them by means of the latter. To keep the rotational speed of the driven rotor of the viscosity coupling, for example the fan, at an optimal value, the quantity of effective shearing liquid supplied to the viscosity coupling is controlled by way of a setting unit, for example a valve.
U.S. Pat. No. 6,079,536 discloses regulating the rotational speed of the driven rotor of a viscosity coupling by means of a regulator that supplies the setting unit with a setting signal dependent on a measured actual rotational speed.
The object of the invention is so to improve known devices and to achieve higher dependability in operation.
This object is accomplished according to the present invention.